There are disclosed herein hole blocking layers, and more specifically, photoconductors containing a hole blocking layer or undercoat layer (UCL) comprised, for example, of a metal oxide, a polymer binder, and a UV light absorber, and which layer can be situated between the supporting substrate and the photogenerating layer. More specifically, there are disclosed herein undercoat or hole blocking layers comprised of some of the components as illustrated in the copending applications referred to herein, such as a metal oxide like a titanium dioxide, and a UV absorber component.
In embodiments, photoconductors comprised of the disclosed hole blocking or undercoat layer enables, for example, undesirable light shock reductions, the minimization or substantially elimination of undesirable ghosting on developed images, such as xerographic images, including improved ghosting at various relative humidity; excellent cyclic and stable electrical properties; minimal charge deficient spots (CDS); and compatibility with the photogenerating and charge transport resin binders, such as polycarbonates. Charge blocking layer and hole blocking layer are generally used interchangeably with the phrase “undercoat layer”.
The demand for excellent print quality in xerographic systems is increasing, especially with the advent of color. Common print quality issues can be dependent on the components of the undercoat layer (UCL). In certain situations, a thicker undercoat is desirable, but the thickness of the material used for the undercoat layer may be limited by, in some instances, the inefficient transport of the photoinjected electrons from the generator layer to the substrate. When the undercoat layer is too thin, then incomplete coverage of the substrate may result due to wetting problems on localized unclean substrate surface areas. The incomplete coverage produces pin holes which can, in turn, produce print defects such as charge deficient spots (CDS) and bias charge roll (BCR) leakage breakdown. Other problems include “ghosting” resulting from, it is believed, the accumulation of charge somewhere in the photoreceptor. Removing trapped electrons and holes residing in the imaging members is a factor to preventing ghosting. During the exposure and development stages of xerographic cycles, the trapped electrons are mainly at or near the interface between the charge generation layer (CGL) and the undercoat layer (UCL), and holes are present mainly at or near the interface between the charge generation layer and the charge transport layer (CTL). The trapped charges can migrate according to the electric field during the transfer stage where the electrons can move from the interface of CGL/UCL to CTL/CGL, or the holes from CTL/CGL to CGL/UCL, and become deep traps that are no longer mobile. Consequently, when a sequential image is printed, the accumulated charge results in image density changes in the current printed image that reveals the previously printed image. Thus, there is a need to minimize or eliminate charge accumulation in photoreceptors without sacrificing the desired thickness of the undercoat layer, and a need for permitting the UCL to properly adhere to the other photoconductive layers, such as the photogenerating layer, for extended time periods, such as for example, about 2,000,000 simulated xerographic imaging cycles. Thus, conventional materials used for the undercoat or blocking layer possess a number of disadvantages resulting in adverse print quality characteristics. For example, ghosting, charge deficient spots and bias charge roll leakage breakdown are problems that commonly occur. With regard to ghosting, which is believed to result from the accumulation of charge somewhere in the photoconductor, consequently, when a sequential image is printed, the accumulated charge results in image density changes in the current printed image that reveals the previously printed image.
Thick undercoat layers are sometimes desirable for photoreceptors as such layers permit photoconductor life extension and carbon fiber resistance. Furthermore, thicker undercoat layers permit the use of economical substrates in the photoreceptors. Examples of thick undercoat layers are disclosed in U.S. application Ser. No. 10/942,277, filed Sep. 16, 2004, U.S. Publication 20060057480, entitled Photoconductive Imaging Members, the disclosure of which is totally incorporated herein by reference. However, due primarily to insufficient electron conductivity in dry and cold environments, the residual potential in conditions, such as 10 percent relative humidity and 70° F., can be high when the undercoat layer is thicker than about 15 microns, and moreover, the adhesion of the UCL may be poor, disadvantages avoided or minimized with the UCL of the present disclosure.
Also included within the scope of the present disclosure are methods of imaging and printing with the photoconductive devices illustrated herein. These methods generally involve the formation of an electrostatic latent image on the imaging member, followed by developing the image with a toner composition comprised, for example, of a thermoplastic resin, colorant, such as pigment, charge additive, and surface additives, reference U.S. Pat. Nos. 4,560,635; 4,298,697 and 4,338,390, the disclosures of which are totally incorporated herein by reference, subsequently transferring the image to a suitable substrate, and permanently affixing the image thereto. In those environments wherein the device is to be used in a printing mode, the imaging method involves the same operation with the exception that exposure can be accomplished with a laser device or image bar. More specifically, the imaging members, photoconductor drums, and flexible belts disclosed herein can be selected for the Xerox Corporation iGEN3® machines that generate with some versions over 100 copies per minute. Processes of imaging, especially xerographic imaging and printing, including digital and/or high speed color printing, are thus encompassed by the present disclosure.
The photoconductors disclosed herein are in embodiments sensitive in the wavelength region of, for example, from about 400 to about 900 nanometers, and in particular from about 650 to about 850 nanometers, thus diode lasers can be selected as the light source.